Our goal is to investigate the effect of advanced age on antiepileptic drug pharmacokinetics and metabolism. Selected elderly and adult male and female ambulant patients taking phenytoin (PHT or CBZ) as monotherapy for epilepsy will be evaluated. The patients will receive a portion of their oral dose of PHT as stable-labeled PHT given intravenously as 100 mg over 5 minutes (one time only). Stable-labeled PHT is NOT radioactive and exhibits the same kinetics and metabolism as unlabeled PHT. Blood samples and urine collections will be analyzed.